1. Field of the Invention
The present invention relates to an image pickup apparatus and method for picking up an observed image obtained by a microscope.
2. Description of the Related Art
An image pickup apparatus for a microscope has been used in which an observed image of the microscope is picked up by image pickup part such as a CCD. For the image pickup apparatus for the microscope, there have heretofore been proposed techniques for obtaining a desired observed image without impairing operability in framing and focusing even under various photographing conditions.
One of the techniques is for use in photographing a dark sample.
When the observed image obtained by the microscope is recorded as a still image, a moving image is usually displayed in a display section for a framing or focusing operation. The image is picked up for an exposure time determined in accordance with brightness of the sample to observe, so that the moving image is displayed with a desired brightness. Therefore, when the observed sample is dark, the exposure time is lengthened so as to obtain an image with the desired brightness.
However, when the exposure time is lengthened in order to pick up the image of the dark sample, a frame rate for obtaining the moving image lowers. Therefore, the operability in framing or focusing is remarkably impaired.
Then, an apparatus disclosed, for example, in Jpn. Pat. Appln. KOKAI Publication No. 11-164191 is known as an image pickup apparatus which is used for photographing the dark sample, that is, a dark subject. In this apparatus, exposure is controlled as follows.
First, an illuminance of the subject is measured. Photometry evaluation values calculated based on the illuminance are compared with predetermined threshold values A to E. Here, A>B>C>D>E. When the photometry evaluation value is not less than the threshold value A, a gain of an amplifier of an image pickup signal is fixed to G0, a diaphragm and exposure time are changed, and an exposure control of an image pickup element is performed. When the photometry evaluation value is less than the threshold value A, the exposure time is fixed to 1/60 second, and an output from the image pickup element is amplified with a predetermined gain. The gain is determined in accordance with a relation between the photometry evaluation value and the threshold value. For example, the gain is G1, when the photometry evaluation value is not less than B and less than A. The gain is G2, when the evaluation value is not less than C and less than B.
According to the method, even when the sample is dark, the frame rate is not impaired. So the operability in framing or focusing is improved.
However, in this method, the brightness of the moving image changes in stages, because a plurality of predetermined gains are used. Therefore, the moving image cannot be displayed constantly with the desired brightness regardless of the brightness of the sample. Moreover, when the photometry evaluation value is less than the threshold value A, the exposure time is fixed at 1/60 second. Therefore, normally, for the subject requiring an exposure time of about one second, since the gain becomes too large, a noise is also amplified with a large gain, image quality is remarkably deteriorated and it becomes difficult to observe the subject.
Moreover, another technique relates to a method for an observer to obtain an intended exposure.
When the observed image obtained by the microscope is recorded as the still image, the exposure control is performed by a closed loop so as to obtain an appropriate level of an image signal. As a result, an optimum exposure time is automatically set. Moreover, an exposure correction value can be set in accordance with a user's preference. In this case, a control set point level is corrected to a level corresponding to the set exposure correction value, and the optimum exposure time is set by the above-described exposure control.
Additionally, the exposure is corrected such that the correction value is added so as to increase the exposure time. In this case, when the corrected control set point level is not less than a predetermined value, the image signal in the closed loop sometimes exceeds a full range. Then, since an image signal level is saturated, and the optimum exposure time cannot be set, the exposure correction intended by a user cannot be performed. Moreover, the correction is performed such that the correction value is subtracted so as to decrease the exposure time. In this case, when the control set point level after the correction is not more than the predetermined value, the level of the image signal lowers and therefore signal-to-noise ratio is deteriorated. As a result, the controlled image quality sometimes lowers.
As a technique for solving the problem, a method disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2000-358189 is known. This method comprises: calculating the optimum exposure time based on the image obtained from the image pickup element; and controlling the exposure in the exposure time. Moreover, when the exposure correction is set, the calculated exposure time is corrected based on the exposure correction value.
According to the method, since the calculated exposure time is corrected in accordance with the exposure correction value, the exposure can be prevented from being influenced by saturation of the image signal in the correction for increasing the correction value, or by a drop of the S/N of the image signal in the correction for decreasing the correction value.
However, this exposure control method is applied, when the image is obtained as the still image. Therefore, for the moving image continuously taken in order to perform the framing or focusing operation, the exposure correction cannot function.